1. Field of the Invention
The present invention relates to an automatic focus adjustment device for use with a camera and that is capable of performing automatic focusing with respect to non-moving and moving subjects (i.e., regardless of subject stability).
2. Description of the Related Art
It is well known that automatic focus adjustment devices have been applied to cameras and other optical devices (e.g., video cameras, etc.) to perform automatic adjustment of the focus state of a photographic or imaging lens (e.g., a taking lens).
Typically, such autofocusing devices utilize a well known technique known as the phase difference method to detect focus displacement (hereinafter "defocus amount") of a subject image from the imaging plane. Based on such a defocus amount, a residual drive amount I that is necessary for focusing (i.e., to move a taking lens or the like) is calculated.
In such autofocusing devices, a calculation of an actuation amount E, which is provided to a drive mechanism of a photographic lens, is based on the following equation: EQU E=a.multidot.i+.beta..multidot..intg.i dt+y.multidot.di/dt (Eq. 1)
using, as the position deviation, the residual drive amount I calculated in the above manner. The coefficients a, .beta. and y, are set as feedback gains in a conventional manner.
Accordingly, with respect to a moving body subject, the proportional term (a.multidot.i) and the differential term (y.multidot.di/dt) are feed back values, and the photographic lens is driven so as to maintain a focused state relative to the speed of the subject image.
Moreover, with respect to a non-moving subject, by way of feedback of the proportional term (a.multidot.i), the photographic lens is driven in a direction which reduces the residual drive amount i, and becomes stationary in the focused state.
In the case of a moving subject, when maintaining the focused state following the movement of the subject, because the proportional term (a.multidot.i) and the differential term (y.multidot.di/dt) become zero, the actuation amount E fed back to the drive mechanism of the photographic lens decreases to a slight or ineffective degree.
When the actuation amount E is reduced to an extremely small degree in the aforementioned manner, due to friction and other energy losses of the drive mechanism, the speed of movement of the photographic lens is a reduced speed, and the movement of the photographic lens becomes minutely delayed with respect to the focus of the subject image.
As such, with respect to a moving subject, it becomes difficult to maintain a correctly focused state. Furthermore, with respect to a moving subject, because the photographic lens is intermittently driven, the smoothness of the focusing adjustment is impaired. Moreover, in the case that the subject is non-moving, when the residual drive amount i becomes rather small, because it is predominantly the differential term (y.multidot.di/dt) which is fed back to the equation Eq. 1, the force acting in the direction of standstill motion excessively slows the drive speed. As such, the response time required for focusing adjustment becomes quite long.
In the prior art, in order to remedy this problem, an integral term (.beta..multidot..intg.i dt) as shown in Eq. 1 was introduced, and by increasing the present actuation amount E using the past residual drive amount i, the time change of the actuation amount E was restrained, and the drive speed of the photographic lens was not abruptly decelerated.
By restraining the deceleration of the photographic lens in the aforementioned manner with respect to a moving body in the focused state following the speed of the subject, the past residual drive amount i was fed back to Eq. 1 by way of the integral term (.beta..multidot..intg.i dt), and the photographic lens was continuously driven in the direction of movement.
Accordingly, while maintaining the focused state, even if the present residual drive amount i becomes rather small, by feeding back the past residual drive amount i, because the photographic lens was continuously driven, the photographic lens was rapidly driven to the focused state, so that the response time for focusing adjustment was shortened.
As such, in prior art automatic focus adjustment devices utilizing the integral term (.beta..multidot..intg.i dt) and because the past residual drive amount i was fed back to the drive mechanism of the photographic lens, the control system included dead-time elements. The problem was that overshooting easily arose in the drive of the photographic lens, and the stability of the system was markedly impaired.
Because of the aforementioned problems, in order to maintain the stability of the camera system, the feedback gains a and y had to be limited, and a problem developed in that it was difficult to obtain good controllability.
Moreover, because a past time residual drive amount i was fed back for later processing, another problem developed in that focus adjustment could not conform to rapid changes of movement of the subject.
Furthermore, in the calculation of the integral term (.beta..multidot..intg.i dt), it was necessary to cumulatively store a past number of periods of the residual drive amount i, and as such, a problem developed in that large amounts of memory (i.e., random access memory) were occupied and required.
Additionally, in the calculation of the integral term (.beta..multidot..intg.i dt), it was necessary to successively add the residual drive amounts i of a number of past periods, and as such, a problem developed in that the load of the calculation process became heavy (i.e., computer processing was onerous due to such repeated addition).